Vasoactive intestinal peptide (VIP), secretin, glucagon, gastric inhibitory peptide (GIP) and, most recently, PHI and growth hormone releasing factor (GRF) are members of a family of peptides which are known to control numerous GI, pancreatic, and endocrine events. Despite their diverse biological activities, considerable sequence homology exists between each peptide thus creating a unique opportunity for efficient (i.e. worthwhile modifications to one peptide might be reasonably expected to be directly applicable to a comparable sequence region of another) structure-activity studies aimed at examining factors responsible for particular biological responses, increasing selectivities, increasing activities, and developing competitive antagonists. Such extensive SAR studies on peptides of this size (ca. 30 residues) have been made possible by our development over a seven year period of rapid solid-phase syntheses and HPLC purification techniques for each of these peptides. Analogs prepared similarly have already yielded more active forms of glucagon and GRF and these design strategies will now be used for VIP and other members of the series. Analogs of these peptides can be expected to have therapeutic significance in many areas, notably diabetes mellitus through the elucidation of new mechanisms governing the control of insulin and glucagon levels and glucagon antagonists, ulcers through secretin effects on bicarbonate release, certain lung disorders through VIP-stimulated bronchodilation, and growth stimulation in cases of hypothalamic GRF deficiencies. Furthermore, competitive antagonists of these peptides, as well as having clinical potentials in some instances, would be of great value in elucidating mechanisms of action of endogenous peptides. Assay methods to be used in this research include effects on GH, prolactin, insulin, and glucagon release in the rat, GH and prolactin release from monolayer pituitary cell cultures, and in vitro effects on adenylate cyclase activity in many tissue types.